Perks

Perks are special abilities and enhancements to your skills. When you level up, you can select one Perk. In order to select a perk, you must meet its skill requirement and have any pre-requsite perks that it requires.

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With the addition of Skyrim Patch 1.9, players can choose to make skills that have reached level 100 "Legendary", which resets the skill to level 15 and returns all the perks, essentially removing the level cap and allowing a character to select all possible perks.